I've never had phone sex, so I wouldn't know how to initiate it.
I would assume you could just ask her. Or maybe start saying sexual things over the phone and maybe it could lead to something. If it's your first time, I don't think you would be that good at it.. just like regular sex. Getting girls hott over the phone is an art. I would suggest practicing it on some of those "whatever" girls you have stashed in your closet, the ones you do all your little experiments on first.
There's also the fact the girl may be completely turned off by even the thought of it. Too risky if you ask me. I would just meeting up with her and just escalate the sexual tension by kino until she gives into REAL sex.